1. Field of the Invention
The present invention relates to a Faraday rotator that is formed of a bismuth-substituted rare-earth iron garnet single crystal and does not use a permanent magnet, the bismuth-substituted rare-earth iron garnet single crystal being used as a Faraday rotator for optical isolators and optical circulators. More particularly, the present invention relates to a method of adding a square hysteresis loop to a bismuth-substituted rare-earth iron garnet single crystal having a compensation temperature near room temperature.
2. Description of the Related Art
Recently, optical fiber communications and optical instrumentation have taken a giant leap forward. Optical communications and optical instrumentation commonly use a semiconductor laser that serves as a signal source. A serious problem associated with a semiconductor laser is a so-called reflected light return where the light is reflected back by the end of the optical fiber and returns to the semiconductor laser. If light reflection return occurs, the oscillation of laser becomes unstable. Therefore, an optical isolator is disposed on the output side of the semiconductor laser to block the light reflection return, thereby stabilizing the oscillation of the laser.
Usually, an optical isolator includes a polarizer, an analyzer, a Faraday rotator and a permanent magnet that causes the Faraday rotator to be magnetically saturated. The Faraday rotator plays a critical role in the optical isolator. The Faraday rotator has a thickness of about several tens microns to about 400 xcexcm and takes the form of a bismuth-substituted rare earth iron garnet single crystal (referred to as BIG hereinafter) grown by liquid phase epitaxy (LPE). Such BIGs include (HoTbBi)3Fe5O12 and (TbLuBi)3(FeAlGa)5O12 etc.
Intensive research have been carried out on rare earth iron garnets as a recording film for a magneto-optic disk and a variety of papers have been reported (Applied Physics. 2., 1973, pp. 219-228; IEEE TRANSACTIONS ON MAGNETICS, VOL. MAG-10, 1974, pp. 480-482; IEEE TRANSACTIONS ON MAGNETICS, VOL. MAG-7, 1971, pp. 397-401; and J. Applied. Physics. 53(3), March 1982, pp. 2754-2758, etc.).
These references disclose methods, principles, and theories of recording, storing, and rewriting data, the methods using the temperature dependence of magnetic properties including the square-shaped hysteresis of rare earth iron garnets.
These references describe a nucleation magnetic field (Hn), which is a measure of stability of the square hysteresis loop of magnetization. Nucleation magnetic field (Hn) is a strength of an external magnetic field applied to a garnet at which the direction of magnetization of the garnet is reversed. When a garnet is place in an external magnetic field in an opposite direction to the direction of magnetization of the garnet and the external magnetic field is increased, a tiny area having a magnetization direction opposite to the direction of the external magnetic field. Then, inversion of magnetization direction is triggered from the tiny area and spreads over the entire garnet body in an avalanche fashion. This tiny area is referred to as nucleation and the field strength of the external magnetic field that gives rise to a nucleation is thus referred to as nucleation magnetic field Hn. One of the above references reports that a garnet chip formed to a size of 1 mm square by etching shows a nucleation field, for example, Hn=1200 Oe, and a garnet having formed to a size of 1 mm square by mechanical scribing shows a nucleation field of 26 Oe, for example. The difference in nucleation field implies that a garnet should be free from defects in shape in order to be magnetically stable.
According to the above references, nucleation field Hn is given by the following equation.
Hn=axc2x7Hs+b/Hs (a and b are proportional constants) The nucleation field Hn diverges as the temperature becomes closer to the magnetic compensation temperature of a garnet material at which the saturation magnetization field Hs of the garnet becomes zero. In other words, closer to a compensation point the temperature is, the larger the nucleation field Hn is. A nucleation field Hn of about several thousand can be observed if a garnet material is nearly ideal. Thus, it appears to be preferable to use a BIG having a compensation temperature near room temperature, as a Faraday rotator that does not use a magnet.
It is not until 20 years after the above references that BIGs having a square hysteresis loop were proposed and put into practical use as a magnet-free Faraday rotator. Japanese Patent Laid-open (KOKAI) No. 06-222311 (Laid open on Aug. 12, 1994) discloses that a BIG having a composition of (GdRBi)3(FeGaAl)5O12 can be used to manufacture, for example, a magnet-free isolator. EP-0 647 869A1 (Published on Apr. 12, 1995) discloses an isolator having a composition of (GdBi)3(FeGaAL)5O12. The isolator incorporates a Faraday rotator sandwiched between glass polarizers, and shows an insertion loss of 0.4 dB and an extinction ratio of 38.8 dB at a wavelength of 1.31 xcexcm. The isolator does not require an external magnetic field.
Japanese Patent Laid-open (KOKAI) No. 09-185027 (laid open on Jul. 15, 1997) discloses an embodiment in which a 100 xcexcm-thick BIG having a composition of Bi1Eu1Ho1Fe4Ga1O12 is cut into a slab of 11.5 mm square and a chip of 2 mm square and the slab and chip have a saturation magnetization 4 xcfx80Ms less than 100 G and shows a square hysteresis loop in the temperature range of xe2x88x9240 to +80xc2x0 C. Although this reference discloses only magnetic properties of the material, the reference describes that the material can also be applied to optical isolators. Patent Preliminary Publication (KOKAI) No. 09-328398 discloses an embodiment in which a garnet has a composition of (TbBi)3(FeAlGa)5O12, a compensation temperature of zero degrees, a square hysteresis loop, a minimum extinction ratio of 38.8 dB, and a minimum external magnetic field of 164 Oe that can reverse the direction of magnetization of the garnet, and therefore the material can be used as an optical isolator.
Magnet-free optical isolators formed of a BIG can be compact and inexpensive because they do not use a permanent magnet. Optical isolators are subjected to an environmental temperatures higher than 100xc2x0 C. during manufacture but the garnets are commonly used at room temperature. Thus, it is desirable that the material is stable in a temperature range centered about room temperature. A geometrical defect of a garnet material is a critical factor that affects the stability of the garnet material. A garnet material free from geometrical defects exhibits a strong resistance to an external magnetic field that is applied to the garnet material in such a direction as to destroy the square hysteresis loop of the garnet material.
The inventors manufactured a wafer-shaped BIG having a magnetic compensation temperature near room temperature in the shape of a wafer. Then, the Faraday rotators were manufactured in the following usual manner. The wafer was cut into chips of a size of about 10 mm square. The chips were lapped to such a thickness that the chip has a Faraday rotation angle of about 45 degrees. Then, anti-reflection coating (AR coating) was applied to both surfaces of the chip. Then, the chips were further cut into desired smaller sizes. The Faraday rotators were magnetized at room temperature (24xc2x0 C.) so that the Faraday rotator acquires a square hysteresis loop. However, most of the magnetized Faraday rotators showed Faraday rotations smaller than they were expected.
This phenomenon is very new and cannot be expected in the usual manner, for example, disclosed in Japanese Patent Preliminary Publication (KOKAI) No. 09-185027, which describes only magnetic properties but suggests applications to isolators. This publication further describes in claim 14 that a BIG preferably has a compensation temperature near room temperature. However, the publication discloses only requirements for optical isolators and makes no mention of a magnetic compensation temperature and specific data of an optical isolator described in the specification. Then, the specification describes on page 6, paragraph 0034, xe2x80x9cto assure magnetic saturation and reproducibility, a magnetic field of 2.5 kOe was used for magnetically saturating all samples of Table 1 except for sample 1, for which 12 kOe was used to insure magnetic saturation.xe2x80x9d In other words, Japanese Patent Preliminary Publication (KOKAI) No. 09-185027 suggests that the use of a sufficiently strong magnetic field ensures magnetic saturation of a material with reliable repeatability.
Thus, the inventors of the present invention assumed that Faraday rotations smaller than they were expected were due to an insufficient magnetic field strength, and therefore the inventors used a magnetic field of about 10,000 Oe to magnetically saturate the garnet material. However, the results remained unchanged. The inventors then demagnetized the chips. Then, the chips were magnetically saturated at 24xc2x0 C. by using a permanent magnet and then the Faraday rotation angles of the chips were measured. Smaller Faraday rotations were observed in more chips when the chips were saturated by using the permanent magnet than when they were magnetized at 24xc2x0 C. to add a square hysteresis loop. Thus, there is a need for a method of manufacturing a Faraday rotator having a desired Faraday rotation angle by using a BIG that has a magnetic compensation temperature near room temperature.
An object of the present invention is to provide a method of manufacturing a magnet-free Faraday rotator by using a BIG that is grown on a non-magnetic garnet substrate by liquid phase epitaxy and has a magnetic compensation temperature in the range of 10 to 40xc2x0 C. A film of BIG is exposed to an external magnetic field higher than 1000 Oe in a direction normal to the major surface of the film at either a temperature at least 20xc2x0 C. higher than the magnetic compensation temperature or a temperature at least 20xc2x0 C. lower than the magnetic compensation temperature, thereby acquiring a square hysteresis loop.
The method is carried out to add a square hysteresis loop to the garnet material after the garnet material has been cut into product sizes.
The square hysteresis loop is preferably added to the garnet material at a temperature that exceeds the compensation temperature by at least 20xc2x0 C.
The square hysteresis loop is preferably added to the garnet material after the garnet material has been assembled into a non-reciprocal device such as an optical isolator and a circulator.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
Embodiment
A BIG of the invention, which is magnetized to have a square hysteresis loop, has a magnetic compensation temperature in the range of 10 to 40xc2x0 C. Such a BIG is usually selected from bismuth-substituted rare earth iron garnet single crystals having a composition of R3-xBixFe5-yAyO12, where R is at least one selected from a group consisting of Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, A is at least one selected from a group of Ga, Sc, Al, and In, and x has the range of 0.7xe2x89xa6xxe2x89xa60.5 and y has the range of 0.5xe2x89xa6yxe2x89xa61.5.
As is well known in the art, the net magnetization of iron (Fe) sites in the crystal of a BIG is opposite in orientation to that of rare earth elements. The magnetization of both iron and rare earth elements increases with decreasing temperature and the temperature dependence of the magnetization of rare earth elements is larger than that of iron at low temperatures. Thus, the net magnetization due to iron site is equal to that of rare earth elements but opposite in orientation at a certain temperature, and therefore the resulting magnetization of the garnet material is zero at that temperature. This temperature is referred to as compensation temperature. Bismuth (Bi) replaces a portion of rare earth elements and increases the Faraday rotation angle of the garnet greatly and therefore is an essential element. The garnet includes bismuth (Bi) as an impurity. The iron site has a magnetization consisting of a group of two iron atoms and a group of three iron atoms, the two groups being opposite in magnetic orientation. Thus, bismuth is an element that substitutes one resultant iron atom to decrease the magnetization resulting from one iron atom. As is clear from the above description, rare earth elements having less temperature dependence can be employed in order to decrease the temperature dependence of magnetization.
For practicing the present invention, a substrate used in liquid phase epitaxy can be of any one of known substrates. Usually, a substrate can be selected from among non-magnetic garnets [(GdCa)3(GaMgZr)5O12] so called SGGG substrate, on the market. The SGGG substrates have lattice constants in the range 1.2490 nm to 1.2515 nm.
In the present invention, a square hysteresis loop is added to a BIG by applying an external magnetic field of higher than 1000 Oe in a direction normal to the major surface of the crystal at a temperature at least 20xc2x0 C. higher than the compensation temperature or at a temperature at least 20xc2x0 C. lower than the compensation temperature. When a square hysteresis loop is added to a garnet material at a temperature at least 20xc2x0 C. lower than the magnetic compensation temperature, the apparatus required will be expensive and large in size. Thus, it is simple and convenient to add a square hysteresis loop at a temperature at least 20xc2x0 C. higher than the compensation temperature. The upper limit of the temperature may be at least 10xc2x0 C. lower than a Curie point of the BIG and a time of about several seconds or longer is enough for sufficiently magnetizing the garnet to add a square hysteresis loop.
A square hysteresis loop is added to the garnet after the material has been cut into desired product sizes. In other words, the garnet material is cut into product sizes and then a square hysteresis is added under a predetermined condition. Then, the physical quantities of the garnet are measured at room temperature. Only those passed examinations are shipped. From a point of view of reliability, magnetization to add a square hysteresis loop to a garnet material should be carried out after the garnet material has been assembled into a non-reciprocal device such as an optical isolator or an optical circulator. The magnetization may be performed before the garnet is assembled into a non-reciprocal device, provided that the square hysteresis loop is not lost due to conditions such as temperature and mechanical stress encountered during the assembly. When the garnet of the invention is applied to, for example, an isolator, polarizing film that serves as a polarizer and an analyzer is bonded to the BIG film and then the entire assembly is subjected to magnetization to add a square hysteresis loop.